This invention relates to an engine exhaust emission control using a nitrogen oxide purification catalyst.
An engine operating with a fuel mixture of an air-fuel ratio in a specific lean region may generate a large amount of nitrogen oxide (NOx) in the exhaust gas. On the other hand, when the engine is operating with a fuel mixture of rich air-fuel ratio immediately after starting, it generates a large amount of hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas.
In order to purify the exhaust gas of the engine, it is necessary to oxidize HC and CO by the oxygen in the exhaust gas such that they are discharged to the atmosphere in the form of water (H2O) and carbon dioxide (CO2). It is also necessary to reduce NOx by HC and CO such that it is discharged to the atmosphere in the form of nitrogen (N2).
In order to purify the exhaust gas of a vehicle engine that runs under a lean air-fuel ratio in the majority of running conditions, it is necessary to process NOx discharged in the aforesaid specific lean air-fuel ratio region. Tokkai Hei 11-62563 published by the Japanese Patent Office in 1999 proposes providing an oxygen storage catalytic converter and a NOx storage catalytic converter in the exhaust passage of such an engine. The NOx storage catalytic converter is provided downstream of the oxygen storage catalytic converter. The oxygen storage catalytic converter is provided with a catalyst which stores oxygen contained in the exhaust gas due to combustion of the air-fuel mixture of a lean air-fuel ratio, and discharges oxygen stored according to decrease of the oxygen concentration in the exhaust gas. The NOx storage catalytic converter is provided with a catalyst which stores NOx contained in the exhaust gas due to combustion of the air-fuel mixture of a lean air-fuel ratio range, and reduces and discharges NOx by a reducing agent component such as HC and CO in the exhaust gas which increase according to decrease of the oxygen concentration in the exhaust gas.
Further, in this prior art device, it is proposed to make the reducing agent component of the exhaust gas increase by supplying additional fuel to the engine after the combustion by ignition is performed by the spark plug of the engine, so as to decrease the NOx storage amount of the NOx storage catalyst and increase the NOx storage capability when the NOx storage amount of the NOx storage catalyst reaches an upper limit. The regeneration of the NOx catalyst by the control of air-fuel ratio in this way is known as NOx catalyst regeneration control.
When unburnt fuel mixes with the combustion gases, it vaporizes and reforms, and the reactivity with NOx is enhanced.
However, when supplying additional fuel after the combustion, it is difficult to keep the vaporizing/reforming state constant, and a large amount of reducing agent component having low reactivity with NOx may be discharged into the air.
On the other hand, it is also possible to increase the fuel supply amount for the combustion so as to increase the reducing agent concentration in the exhaust gas. In this case, although the reforming state of unburnt fuel is better than in the case where additional fuel is supplied after the combustion, the air-fuel ratio of the air-fuel mixture which bums is enriched due to the increase of the fuel supply amount. When the fuel part of the air-fuel mixture which is burnt becomes excessive, i.e., if there is too much enrichment of the air-fuel ratio, the engine will misfire, so there is a limit to how much the fuel supply amount for the combustion can be increased. If the fuel supply amount for the combustion is merely increased within this limit, regeneration of the NOx storage catalyst takes a long time.
Moreover, if a large amount of oxygen has been stored in the oxygen storage catalytic converter, it reacts with CO or HC in the exhaust gas, so the HC or CO amount flowing into the downstream NOx storage catalytic converter will not increase until the release of oxygen in the oxygen storage catalytic converter stops. Therefore, the regeneration of the NOx storage catalyst takes a long time, and since the engine has to run under a rich air-fuel ratio throughout the regeneration operation, fuel-cost performance of the engine is impaired.
It is therefore an object of this invention to perform regeneration of the NOx storage catalyst efficiently in a short time.
It is a further object of this invention to prevent discharge of noxious substances during the regeneration of the NOx storage catalyst.
In order to achieve the above objects, this invention provides a control device for such an engine that comprises a fuel supply mechanism which supplies main fuel to generate an air-fuel mixture for combustion, an exhaust passage which discharges exhaust gas generated by the combustion of the air-fuel mixture, a first catalyst disposed in the exhaust passage which stores oxygen contained in the exhaust gas and releases stored oxygen for oxidizing a reducing agent component contained in the exhaust gas, and a second catalyst disposed downstream of the first catalyst in the exhaust passage which stores nitrogen oxides in the exhaust gas and releases stored nitrogen oxides by reducing the stored nitrogen oxides by the reducing agent component contained in the exhaust gas.
The control device comprising a sensor which detects a running condition of the engine and a microprocessor programmed to calculate a storage amount of nitrogen oxides by the second catalyst based on the running condition, determine whether or not the storage amount is greater than a predetermined value, control the fuel supply mechanism to supply additional fuel to the engine after the combustion of the air-fuel mixture generated by the main fuel when the storage amount exceeds the predetermined value, determine, after a supply of the additional fuel to the engine has started, whether or not a predetermined condition is satisfied, and control the fuel supply mechanism to stop supply of the additional fuel and to increase an amount of the main fuel when the predetermined condition is satisfied.
This invention also provides a control device comprising a first mechanism for detecting a running condition of the engine, a second mechanism for calculating a storage amount of nitrogen oxides by the second catalyst based on the running condition, a third mechanism for determining whether or not the storage amount is greater than a predetermined value, a fourth mechanism for controlling the fuel supply mechanism to supply additional fuel to the engine after the combustion of the air-fuel mixture generated by the main fuel when the storage amount exceeds the predetermined value, a fifth mechanism for determining, after a supply of the additional fuel to the engine has started, whether or not a predetermined condition is satisfied, and a sixth mechanism controlling the fuel supply mechanism to stop supply of the additional fuel and to increase an amount of the main fuel when the predetermined condition is satisfied.
This invention also provides a control method comprising detecting a running condition of the engine, calculating a storage amount of nitrogen oxides by the second catalyst based on the running condition, determining whether or not the storage amount is greater than a predetermined value, controlling the fuel supply mechanism to supply additional fuel to the engine after the combustion of the air-fuel mixture generated by the main fuel when the storage amount exceeds the predetermined value, determining, after a supply of the additional fuel to the engine has started, whether or not a predetermined condition is satisfied, and controlling the fuel supply mechanism to stop supply of the additional fuel and to increase an amount of the main fuel when the predetermined condition is satisfied.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.